The invention relates to a squirrel-cage rotor for an asynchronous machine. Furthermore, the invention relates to a process for producing a squirrel-cage rotor.
Such a squirrel-cage rotor is known from DE 43 08 683 A1. This rotor comprises a laminated core with slots, into which short-circuited bars made of copper are initially pushed in. At the end faces, the copper bars are bonded to one another by short-circuiting rings. These short-circuiting rings are produced from aluminum by a die-casting process. As the short-circuiting rings are being cast, at the same time the residual cross section which remains in the slots around the inserted copper bars is filled with aluminum, such that the short-circuiting rings are bonded to the die-cast bar parts formed in the residual cross section.
Since copper has a very much higher electrical conductivity than aluminum, it is thereby possible to achieve a decisive improvement in the efficiency of such an asynchronous machine compared with an asynchronous machine with a squirrel-cage rotor consisting entirely of die-cast aluminum material.
During operation, the asynchronous machine and therefore also the squirrel-cage rotor pass through thermal cycles which are dependent on the electrical losses within the machine. This leads to thermal expansion in the rotor which is inhomogeneous in such a squirrel-cage rotor on account of the differing coefficients of thermal expansion of aluminum and copper. As a result, the copper bars can break away from the die-cast aluminum material of the short-circuiting rings, as a result of which finally the electrical transition conductivity between the short-circuiting rings and the rotor bars is impaired.
It is possible to realize efficiencies similar to those for the above-described squirrel-cage rotors produced by means of aluminum die-casting processes using a die-cast copper rotor. Since this consists entirely of copper, the aforementioned problems relating to the differing coefficients of thermal expansion do not arise. However, the copper die-casting process is very complex and places stringent requirements on the die-casting mold, since copper has to be heated to temperatures of above 1000° C. for the die-casting process. In the case of aluminum, by contrast, it is possible to work at very much lower temperatures during die-casting. Furthermore, a squirrel-cage rotor which consists entirely of copper is heavier and therefore more inert in terms of its starting response than the squirrel-cage rotor known from DE 43 08 683 A1, which has short-circuiting rings made of aluminum.
The invention is based on the object of increasing the electrical efficiency for a squirrel-cage rotor which consists of two materials and is produced by means of die-casting processes.